This invention relates to the measurement of the thickness of an object. In particular, this invention relates to a device for and a method of measuring not only the overall thickness of an object but also the thickness of a thin film formed on the surface of an object, as well as the thickness of each layer of a multi-layered object.
It is a common practice to form a painted film on the surface of an automobile body or the like in order to prevent the rust generation or material degradation due to corrosion or to improve its external appearance. Polarization films and anti-reflection films are surface-coated for changing the optical characteristic of the film. It is important to control the thickness of such films because it affects not only the characteristics of the film itself but also the characteristics of the object on which the film is formed.
Conventional methods for measuring the thickness of a coated film include the displacement measurement method which makes use of a displacement sensor and the quantitative measurement method by which the change in mass of an object is measured before and after a film is formed. The former is a method of calculating the thickness of a film from the position of the surface of the object before the film is formed and the position of the surface of the formed film. Since it requires the technology of accurate positioning because the reference surface of the displacement sensor and the surface of the object must always be positioned carefully for making a comparison and since a measurement must be made both before and after the film is formed, it is a cumbersome method. The latter is a simpler method because only the mass of the object is measured before and after the film is formed and no very accurate technology not required but it is not capable of an accurate measurement if the film is much lighter or thinner than the object on which it is formed.
In view of the problems of such prior art methods, Japanese Patent Publications Tokkai 52-18591 and 2000-165369, as well as Japanese Patent 3019510, for example, have disclosed a new technology for measuring the thickness of a film simply and accurately by generating a resonance within the film and measuring its thickness based on its frequency and the speed of sound through the material of the film.
According to the technology of Japanese Patent Publication Tokkai 52-18591 (Ref. 1), an ultrasonic probe in contact with an object is vibrated in order to inject ultrasonic waves into the object. Thus, the resonance frequency of the film can be obtained by varying the frequency of vibrations of the probe to change the frequency of the incident ultrasonic vibrations, and the film thickness can be calculated from the resonance frequency thus obtained. If the thickness of the portion contacted by the probe can be measured, the thickness of any portion can be obtained reliably.
According to the technology of Japanese Patent 3019510 (Ref. 2), pulsed ultrasonic waves are injected from a probe in contact with the object. The resonance frequency of the object can be obtained by analyzing the vibrations of the object caused by the injected ultrasonic waves, and the thickness of the film can be obtained from this resonance frequency. If the thickness of the portion contacted by the probe can be measured, the thickness of any portion can be obtained reliably.
According to the technology of Japanese Patent Publication Tokkai 2000-165369 (Ref. 3), an eddy current is caused inside the film by means of an electromagnetic ultrasonic sensor and vibrations are caused inside the film by mechanical vibrations of the film caused by this eddy current. Since the resonance frequency of the film can be obtained by analyzing the vibrations thus generated inside the film, the thickness of the film can be obtained by one measurement as by the technology of Ref. 2.
The technologies of Refs. 1 and 2 allow the thickness of the contacted portion of a film by the probe to be accurately measured but are likely to cause contamination or damage to the contacting surface portion of the object to be measured and hence these methods cannot be used, for example, on the surface of a carbon crucible of an oven for lifting single crystalline silicon which is processed with a black carbon material because, if there is a contaminant attached to this carbon film, the interior of silicon becomes contaminated as the single crystalline silicon is grown. When it is desired to measure the thickness of a film extending over a large area such as the film coating of an automobile body, furthermore, the ultrasonic probe for these methods will have to be moved over the entire automobile body and this makes it difficult to arrange and handle the cables connected to the probe.
The technology of Ref. 3 is convenient because it is capable of generating an eddy current inside a film without contacting its electromagnetic ultrasonic probe directly to the film and hence of measuring the film thickness in a non-contact mode but it is difficult to limit the region in which the eddy current is generated. Moreover, since the measured thickness is an average value over the region where the eddy current is generated, it is not possible to measure the thickness of a localized portion. Since an eddy current must be generated, furthermore, this method is applicable only to the measurement of thickness of a film of a ferromagnetic or electro-conductive material, that is, the thickness of a film of other materials cannot be measured by this method. Still another problem of this method is that the frequency of the vibrations that can be generated within a film by an electromagnetic ultrasonic sensor is at most about 10 MHz. Films with thickness about 0.1 mm may be measurable theoretically but it is difficult to measure a film thickness less than 5 mm as a practical matter. Since a disproportionately large amount of energy must be inputted to the sensor compared to the energy of vibrations that are generated in the film, a large current is required for the sensor and a large-size equipment will be required.